Green’s Dictionary of Slang

a euph. for Christ’s Cross, e.g. so help me crisscross!

1887		H. Frederic Seth’s Brother’s Wife 324: S’elp me die, crisscross, I ain’t!
1909		J. Ware Passing Eng. of the Victorian Era 98/2: Criss-cross (Peoples’). A corruption of Christ’s Cross – one of the few religious exclamations which have not become vulgarized since the Reformation [...] Sometimes used exclamatively. ‘S’help me criss-cross’ (or crass), ‘I didn’t!’ Not many who use this word appear to have any idea, of its meaning, yet it is one of the few old Catholic oaths which have retained much of the original sound.